The present invention relates to frequency reuse in a mobile communications system, and more particularly to assigning a channel, associated with one of a number of fixed reuse plans, to a mobile call as a function of the mobile unit type, the communication data service being requested by the mobile unit, and base station receiver detection and transceiver processing capabilities.
It is known that the frequency reuse plans of a cellular communications system affect both the Carrier-to-Interference (C/I) immunity and the traffic-carrying capacity of the system. For example, a high frequency reuse plan (i.e., a plan that employs shorter reuse distances) will improve the traffic-carrying capacity of the system; however, it will also result in a degradation in the C/I performance. In contrast, C/I performance can be improved by utilizing a lower frequency reuse plan (i.e., a plan that employs longer reuse distances), but this improvement is achieved at the expense of the traffic-carrying capacity of the system.
In addition, downlink performance for one mobile unit versus another, for a given link quality (e.g., a C/I condition), is not necessarily uniform. That is because downlink performance varies as a function of the mobile unit type, that is, the type of technology employed by each mobile unit. For example, there are now mobile units that employ enhanced receiver detection features such as receiver diversity and interference cancellation techniques. These mobile units are referred to hereinbelow as "enhanced" mobile units. These enhancements permit the mobile unit to adequately function even if the downlink is operating at lower than normal link quality conditions. Consequently, enhanced mobile units may be capable of receiving information over channels that are allocated to a high frequency reuse plan.
To employ a high reuse plan and to increase the traffic-carrying capacity of the system, the base station connected to the mobile unit must also be capable of operating at the same lower than normal link quality condition. This can be accomplished by providing similar receiver detection enhancements at the base station (e.g., receiver diversity and interference cancellation techniques). However, until all mobile units employ these enhancements, mobile communications systems must continue to support conventional mobile units (i.e., mobile units that do not possess any of the aforementioned receiver detection enhancements), which cannot adequately operate at lower than normal link quality conditions. The problem, of course, is that if a system operates at higher link quality conditions to satisfy the requirements of conventional mobile units, it cannot take advantage of the enhanced mobile units and increase the traffic-carrying capacity.
Furthermore, mobile units are utilized for more than simply low bit rate voice communication. For example, mobile units are often used for high bit rate data transmission. Compared to low bit rate voice communication, high bit rate data transmission is more susceptible to interference. The degree to which the transmission is susceptible may depend upon the presence or absence of various processing enhancements such as error correction techniques (e.g., forward error correction coding) and multi-level modulation techniques. While more extensive error correction techniques tend to reduce the susceptibility of the link to interference, the error correcting codes employed to implement the error correction take up precious bandwidth. Consequently, error correction coding is often minimized so that a greater percentage of the bandwidth can be dedicated to data transmission. In any event, to support high bit rate data services, a system may be forced to operate at higher than normal link quality conditions.
A number of documents describe techniques for overlaying multiple reuse patterns within a given geographical area. For example, U.S. Pat. No. 4,144,411 to Frenkiel discloses a cell splitting technique in which there is static reuse of frequencies in a large-cell type reuse pattern to a miniature-sized overlaid, but same type reuse pattern. An article by Samuel W. Halpern entitled "Reuse Partitioning in Cellular Systems", presented at the 33rd IEEE Vehicular Technology Conference on May 25-27, 1983 in Toronto, Ontario, Canada describes a cellular system having multiple reuse levels within a given geographical area whereby, for example, a cluster of cells normally employing a seven-cell reuse pattern may simultaneously operate on a three-cell reuse pattern. In this scheme, one set of frequencies is dedicated to the three-cell reuse pattern while another set of frequencies is dedicated to either a seven-cell or a nine-cell reuse pattern. By allocating frequencies from both sets for use in a particular cell, one cell site may operate on either reuse pattern.
As a variation on the static reuse schemes described in the Frenkiel and Halpern documents, U.S. Pat. No. 5,257,398 to Schaeffer describes a system employing dynamic multiple reuse levels. In this scheme, carriers are allocated according to one reuse pattern during one time interval, and allocated according to another reuse pattern during another time interval.
U.S. Pat. No. 5,038,399 to Bruckert describes a method for assigning channel reuse levels in any of the aforementioned multi-reuse level cellular systems.
Although the above mentioned documents describe techniques for establishing and utilizing multiple reuse plans in a single geographical area, each makes a channel or reuse plan assignment based on measurements of existing C/I conditions at the time the assignment is to be made. In effect, these techniques treat all mobile units as having equivalent performance characteristics for any given C/I condition. Moreover, they ignore the fact that a mobile unit may request one of a number of different communication services, each of which may have a distinctly different effect on minimum required link quality. Therefore, these conventional techniques ignore any possible traffic-carrying capacity gains that could be achieved by considering the mobile unit type, the base station capabilities, and the type of communication service being requested by the mobile unit.
U.S. Pat. No. 5,666,649 discloses one approach to improving system performance when a mixture of enhanced and non-enhanced mobile units are present in a communications system. In particular, Dent discloses detecting whether the mobile unit has a particular capability to receive signals at a lower signal-to-interference ratio, and operating the base station to improve system performance in response to detection that the mobile unit has the particular capability. As described by Dent, improving performance includes reducing the consumption of spectral resources such as, for example, by adjusting the power level of the frequency channel to reflect the minimum required link quality (i.e., C/I condition) of the mobile unit, thus permitting the system to increase the traffic-carrying capacity by increasing the reuse of the frequency channel. U.S. patent application Ser. No. 08/299,420 is incorporated herein by reference.
There is, therefore, a need for a communications system that is suitable for use by non-enhanced mobile units, which also takes into account the presence of enhanced mobile units that employ one or more of the above-identified feature enhancements, the capabilities and limitations of the base station, and the communication services being requested by the mobile unit, when assigning a channel associated with one of a number of fixed reuse plans, so as to increase the system's traffic-carrying capacity.